Understanding the characteristics of water adsorption in zeolitic imidazolate framework-derived porous carbon materials

Zeolitic imidazolate framework (ZIF)-derived porous carbons are a new subclass of N-doped porous carbon materials showing great potential for many applications. However, water vapor is present in various industrial applications, and previous studies have tended to neglect water adsorption on ZIF-derived porous carbons. In this study, ZIF-8-derived porous carbon materials exhibiting different surface chemistry and pore structure have been prepared, and used to characterize the behavior of water adsorption. Investigations suggest that water uptake at P/P-0 < 0.2 is positively correlated with the N content, and that pyridinic-N and pyrrolic-N species make the dominant contribution to this water uptake. Strong polarization of surface structure occurs in pyridinic-N and pyrrolic-N species, which provides strong electrostatic interactions between these species and water, leading to an increase in the low-pressure adsorption capacity. As the pressure is increased, the water adsorption process becomes highly dependent upon the porous structure, and there is a critical pore width above which pore-filling occurs. Our results indicate that water uptake at P/P-0 = 0.6, 0.8, and 0.9 is indeed directly proportional to cumulative pore volume with pore widths of the 12, 22 and 40 angstrom, respectively. Thus, the small micropores of < 12 angstrom are filled first at P/P-0 < 0.6, followed by the larger micropores of 12-22 angstrom at 0.6 < P/ P-0 < 0.8, and finally the mesopores of 22-40 angstrom at 0.8 < P/P-0 < 0.9. The results presented in this work should be useful for the selection and construction of ZIF-derived porous carbon materials for applications where water is involved.